Hairpin-type electric resistance heating element

ABSTRACT

An electric resistance heating element of the hairpin type is provided by a molybdenum disilicide resistance wire loop transversely connected to molybdenum disilicide terminals. The resistance wire is sandwiched between the ends of two pieces of terminal wire and the ends of these pieces are butt welded together on the resistance wire, to form each terminal.

BACKGROUND OF THE INVENTION

Electric resistance heating elements made of molybdenum disilicide areconventionally made in the form of an elongated loop of U-shape, ofmolybdenum disilicide resistance wire having legs to the ends of whichmolybdenum disilicide terminals are butt welded. They are commonlycalled hairpin elements.

The terminals are in the form of molybdenum disilicide wire and are madesubstantially larger in diameter than the resistance wire to reduceterminal heating when the element is in service. Electric resistancebutt welding is used to weld the terminal wire ends to the resistancewire ends and this requires tapering of the terminal ends to provideends having a diameter substantially the same as that of the resistancewire so that the butt welding is facilitated. The terminals are madelong enough to accommodate their clamping by clamps which apply theelectric power to the element.

Molybdenum disilicide is extremely hard, and to taper the terminal ends,requires the use of abrasive grinding techniques. The material is alsobrittle and this grinding is not only inherently expensive, but alsoinvolves the risk of terminal breakage.

The terminals are welded in axial alignment with the hairpin wire legs,and the terminals are vertical when clamped under service conditions sothat the hairpin or loop of resistance wire depends vertically to avoidsagging problems when the element is in operation. However, for someapplications it is desirable to mount the terminals horizontally withthe loop of resistance wire depending, but this necessitates rightangular bending of the terminal wire, because it has heretofore beenimpossible to weld the resistance wire right-angularly to the largerterminal wire.

The use of bent terminals requires an undesirable extent of terminalwire in the furnace using the element.

Molybdenum disilicide electric resistance wire is sold in a wide rangeof diameters under the trademark "KANTHAL SUPER".

DESCRIPTION OF THE INVENTION

This invention is a new heating element of the described type. Themolybdenum disilicide resistance wire is angularly, such as rightangularly, welded to the molybdenum disilicide terminal wire by beingsandwiched between the ends of two separate pieces of the terminal wirewith the ends of these pieces butt welded together with the resistancewire welded therebetween. The two pieces can be of the samecross-sectional area so that butt welding becomes possible.

The butt welding can be by the usual electric resistance butt weldingmethod. To make this practice possible, for each terminal connection twoterminal wires of substantially the same cross-sectional area, ordiameter, are used, the resistance wire of small cross-sectional area,or diameter, being transversely positioned between the terminal wireends. When desired, the transverse arrangement may be right angular.Ordinarily the terminal wires have the same contour throughout theirlengths, and are cylindrical. Neither of the terminal wires need beground to taper them to provide a cross-sectional area corresponding tothat of the resistance wire, so the problems connected with grindingmolybdenum disilicide are eliminated.

The orientation of the resistance wire relative to the terminal wires isinherently a transverse relationship which can be right angular, withoutbending of either wire. When the finished element is put in service, theterminal wires can be horizontally positioned by terminal clamps whichboth mount and power the element, with the resistance wire loop orhairpin depending vertically.

For the butt welding, with the resistance wire sandwiched, orpositioned, between the ends of the two terminal wires, which should beaxially aligned with each other, both of the two terminal wires can beeasily clamped by a butt-welding machines's clamps, because they may bemade relatively long for easy engagement by any conventionalbutt-welding machine's clamps which in this case as usual serve to bothpush the wire ends together and conduct the electric power required forthe heating and butt welding of the two molybdenum disilicide terminalwires. One terminal wire can be given the length required by thefinished terminal for easy operational clamping and mounting of theelement, and the other terminal can be of any length providing it islong enough for clamping by the butt-welding clamp. After welding, thelast-mentioned terminal wire can be cut very close to the butt weld sothat it is left as only a short stud. The last-mentioned wire could bevery short or in the form of a molded and sintered button, if backed-upby a carbon rod clamped by the welding machine clamp and removed afterthe welding. Then the cutting would be eliminated.

The wire used for the foregoing can be selected from commerciallyavailable round molybdenum disilicide wire which is necessarily straightbecause its brittleness prevents bending when cold. The wire materialcould be other refractory metal silicides if such becomes available.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are only to illustrate in a schematic way theprinciples of this invention, the various figures being as follows:

FIG. 1 is a vertical section of a small or miniature furnace such asmight be used by a dentist when making gold inlays;

FIG. 2 is a cross section taken on the line II--II in FIG. 1;

FIG. 3 shows the two pieces of terminal wire positioned end-to-end withthe resistance wire positioned between, this being a side elevation;

FIG. 4 is the same as FIG. 3 but is a top view;

FIG. 5 is like FIG. 4 but illustrates the butt welding operation; and

FIG. 6 shows the finished result while illustrating that the excesslength of the one terminal wire may be cut off.

DETAILED DESCRIPTION OF THE INVENTION

The small or miniature furnace illustrated by FIGS. 1 and 2 to exemplifythe principles of this invention is a type that has heretofore been madenecessarily with openings in its roof or top for passage of the usualterminals of a hairpin element, having the terminals in axial alignmentwith the legs of the resistance wire loop or hairpin and with theterminal ends tapered and butt welded to the resistance wire ends aspreviously described, all the parts made of molybdenum disilicide. Thisis a disadvantage because upward heat loss is involved.

Using the present invention, the molybdenum disilicide resistance wireloop 1 depends as usual but the butt welded molybdenum disilicideterminals 2 extend horizontally. This permits the top of the furnaceenclosure 3 to be completely closed and a permanent part of theenclosure. For replacement of the element when necessary, a side door 4is provided the furnace enclosure, permitting lateral or transverseremoval and replacement of elements. Side openings provide for passageof the terminal 2 to terminal clamps 5 on the outside of the enclosure.This arrangement of furnace and element parts can be obtained when usingthe prior art end-to-end terminal-to-resistance wire arrangement only bymaking right angular bends in the terminal wire.

When making each terminal arrangement of the present invention, FIGS. 3and 4 show how the resistance wire 1 has its upper end positionedbetween the two terminal wires 2a and 2b, each of which are clamped bythe jaws 6 of an electric resistance butt-welding machine. For anexample, and assuming a small furnace element, the resistance wire 1 maybe round molybdenum disilicide wire having a diameter of 1.5 mm whileboth of the terminal wire parts 2a and 2b may be 3 mm diametermolybdenum disilicide wire. The wires can be cylindrical wires free fromshaping and in the commercially obtainable form.

FIG. 5 shows the butt-welding operation in progress, the two jaws 6being powered by suitable current as indicated at 7 and pushed togetheras indicated by the arrows so that the two ends of the terminal wire arebutt welded together and about the terminal wire 1. The terminal wireitself can join in so that a welded mass is obtained, the two terminalwire ends upsetting as indicated in exaggerated form in FIG. 5.

During this butt welding operation, the two terminal wires 2a and 2bare, of course, axially aligned with each other, but neither need beshaped by a grinding operation as was required before. The resistancewire can depend right angularly from the terminal wires without bendingof any of the wires. To emphasize that no terminal grinding operationsare involved, it can be said that both wires are cylindrical throughout.Joint designs, however, can be applied to the larger diameter with adiamond saw.

Assuming the terminal wire length or piece 2a is to be the terminal, itcan be provided with an appropriate length for use as indicated by FIGS.1 and 2, for example. However, the other terminal part or piece 2b maybe of any convenient length. Both 2a and 2b should be long enough forclamping by the clamp 6. FIG. 6 serves to illustrate how to finish eachterminal, the wire 2b is cut by a diamond saw, for example, so as toleave only a very short piece 2c. This is in the form of a short studonly.

In the foregoing way, the electric resistance wire loop or hairpin 1 isprovided with the right angle terminals 2 shown by FIGS. 1 and 2, withthe attendant advantages previously noted.

Although the small or miniature furnace has been used to exemplify thepresent invention, the principles involved should not be understood asbeing limited with respect to size. Hairpin elements provided with rightangular terminals may be applicable to large industrial furnaces usingeven the largest size of prior art molybdenum disilicide hairpinelements.

In any instance, the terminals and resistance wire loop need notnecessarily be at right angles to each other.

What is claimed is:
 1. An electric resistance element comprising arefractory metal silicide resistance wire, a terminal comprising a firstlength of refractory metal silicide wire having a uniform contour andcross-sectional area through its extent, said resistance wire beingpositioned transversely across one axial end of said first length, and asecond length of refractory metal silicide wire having one end abuttingsaid end of said first length and said resistance wire sandwichedbetween said ends and the latter electric resistance butt weldedtogether and thereby integrated with said resistance wire, said lengthshaving a contour and cross-sectional area substantially identical witheach other and being axially aligned with each other, saidcross-sectional area of said lengths being larger than thecross-sectional area of said resistance wire.
 2. The element of claim 1in which said resistance wire has a hairpin shape having two straightinterspaced legs extending from a loop interconnecting the legs, each ofsaid legs having an end having one of said terminals extending rightangularly therefrom and the terminals being parallel to each other. 3.The element of claims 1 or 2 in which said second length of saidterminal wire is shorter than said first length of said terminal wire,the latter being adapted for operational clamping of said element.